A wireless local area network (WLAN), such as for example, (but not limited to), an IEEE 802 WLAN, uses a carrier-sense multiple access/collision avoidance (CSMA/CA) scheme to share a wireless medium (WM) among a plurality of wireless transmit/receive units (WTRUs), (i.e., mobile stations (STAs)). This scheme forces the WTRUs to sense whether or not the WM is idle or busy before transmitting a packet. This is performed using a clear channel assessment (CCA) function. If a WTRU senses that the channel is busy, (later referred to as a WTRU hearing another transmission), the WTRU cannot transmit its packet and has to wait until the channel is idle before trying to win contention of the channel. This is referred to as deferring. By preventing two WTRUs which hear each other from transmitting simultaneously, the CSMA/CA scheme controls the level of interference in the system.
A low level of interference in a WLAN indirectly improves the capacity of the system as the throughput achieved on each radio link increases as interference decreases. On the other hand, the costs associated with preventing two nodes that hear each other from transmitting simultaneously on the same channel are realized in that scarce spectrum channel resources cannot be reused, thus leading to capacity constraints in systems where multiple base station systems (BSSs) use the same channel.
In CSMA/CA systems, the determination of whether or not a first WTRU defers to a second WTRU, (where the second WTRU is located at a given path loss from first WTRU), depends on two factors: (1) the transmission power used by the first WTRU; and (2) the reception/deferring thresholds used by second WTRU. It is therefore possible to control the level of deferral, and thus the capacity, in a WLAN by controlling either one or both of these factors. Two key reception/deferral thresholds may be used in the CCA functions of WTRUs operating in CSMA/CA, which are: (1) the energy detect threshold (EDT), which represents the smallest received signal power for which reception of a packet will be attempted; and (2) the defer threshold (DT), which represents the smallest received signal power for which transmission of a packet by the WTRU will be deferred.
In summary, it is sometimes better from the point of view of capacity to decrease the transmission power and/or to increase the DT/EDT of WTRUs in co-channel BSSs such that the WTRUs from one of the BSSs do not defer to those of another one of the BSSs, and vice-versa. However, problems may arise if the adjustment of deferral parameters is performed individually at each WTRU without consideration for the effect of this adjustment to other WTRUs. For instance, if a WTRU reduces its transmission power excessively, it can become impossible for other WTRUs in the same BSS to detect the signal from this WTRU above the EDT or DT, and a hidden node situation would arise, resulting in degraded capacity. Hidden nodes phenomena across BSSs may also occur. For the optimization to work properly, coordination between the WTRUs is needed to allow gathering all the required information at the node making the decision on the setting of the deferral parameters. This node could be the WTRU itself or an access point (AP) to which the WTRU is associated. In the latter case, coordination is also needed to allow the AP to communicate the values of the optimal deferral parameters to the concerned WTRU(s).